The conductor/oxide semiconductor field effect transistor (MOSFET) has been known in the art for some time. MOSFETs can be used in a variety of applications, most importantly in semiconductor memory devices. The typical MOSFET consists of a substrate of a specified conductivity type with two heavily doped regions, of an opposite conductivity type, formed in its face. The heavily doped regions, designated the drain and the source, are separated by a channel region. A thin oxide layer is on the surface of the channel and a "metal" (typically highly-doped polycrystalline silicon) is deposited on it to form the gate.
In a typical induced channel MOSFET, when the gate is left floating or a very small voltage is applied to the gate, the path between the drain and the source represents two series diodes back to back, which precludes current flow. When an adequate positive voltage is applied at the gate (NMOS), however, electrons are attracted from the substrate and accumulate at the surface beneath the gate oxide layer, inducing a conductive channel between the drain and the source and allowing current to flow. To attract sufficient numbers of electrons to form such a channel, the voltage applied to the gate must be equal to or greater than a threshold value V.sub.t.
The formation of the source and drain diffused region requires several steps. A conventional set of these steps includes defining the areas of the substrate in which the diffusion will be performed by depositing and patterning a photoresist on the overlying oxide layer and then etching the exposed oxide away. Then, following definitions of the boundaries of the prospective diffused regions, the actual implant must be made. Thus, by eliminating the need to create the heavily doped diffused source and drain regions, the process of manufacturing similar transistors can be reduced.
The advantages of reduced process steps are greatly magnified when an array, such as a memory array, of MOSFETs is being contemplated. The elimination of the source and drain diffusions (or implants) will allow the array to be more scaleable, with a consequent improvement in memory cell density. Further improvement in cell density can be achieved if adjacent cells can be isolated from each other electrically without intervening physical structure, such as field oxide.